Apparatus for the thermal decomposition of saturated hydrocarbons to produce unsaturated hydrocarbons



TED

Dec. 10, 1968 M. FREUND ETAL APPARATUS FOR THE THERMAL DECOMPOSITION OF SATURA HYDROCARBONS TO PRODUCE UNSATURATED HYDROCARBONS Filed June 2. 1965 Fig. I

Fig. 2

INVENTOR M/HHLY, FREUND ZOLTHN NR6! 5 LA'sLLO S ZEPfSY United States Patent 01 hoe 3,415,628 Patented Dec. 10, 1968 3,415,628 APPARATUS FOR THE THERMAL DECGMPOSI- TION F SATURATED HYDRUCARBONS T0 PRODUCE UNSATURATED HYDRGCARBONS Mihrily Freund, Zoltan Nagy, and Lassie dzepesy, Budapest, Hungary, assignors to Magyar Asvanyolaj es Foldgaz liiiserleti intezet, Veszprem, Hungary, a firm Filed June 2, 1065, Ser. No. 460,718 Claims priority, application Hungary, June 4, 1964, MA 1,350, MA 1,351 6 Claims. (Cl. 23-277) ABTRACT OF THE DHSCLUSURE Apparatus for the flame reaction of a combustible gaseous mixture includes a diffuser having the gaseous mixture flowing longitudinally therethrough, a combustion chamber wherein the gaseous mixture is burned and an intermediate section connecting the diffuser to the combustion chamber. The intermediate section is formed by a series of laterally spaced substantially parallel plates forming restricted flow channels therebetween. These plates have parallel spacer ribs and each rib has a groove extending longitudinally of its outer edge and the grooves form additional flow passages which are restricted relative to the flow in the channels between the plates.

It is known that unsaturated hydrocarbons or a mixture thereof may be prepared by thermal decomposition of saturated hydrocarbons or a mixture of the same. By cracking not only unsaturated hydrocarbons and saturated hydrocarbons of lower alkyl chains, but hydrogen, carbon, carbon monoxide, carbon dioxide, oxygen, oxygencontaining compounds and further oxidation products are also formed.

Cracking is carried out at a temperature of 800 to 1600 C. and for a very short period of time to 10- seconds), but it requires significant heat consumption. Therefore very great heat quantities are to be transferred to the raw material to be decomposed in a very short time and this comprises the greatest difficulty of cracking.

Heat transfer is often carried out by mixing the hydrocarbon to be decomposed with oxygen or other oxidizing gases in such ratio that partial burning takes place. The heat quantity thus formed is directly transferred to the unburnt part of the raw material, thus providing the heat requirement of thermal decomposition. A wellknown example of the above method is the partial oxidation of methane to give acetylene and synthesis gas.

According to the process, generally a mixture consisting of 62% of methane and 38% of oxygen, preheated to 500-600" C., is partially burnt. The reaction takes place at a temperature above 1400 C. and, on sudden quenching of the same, a mixture containing 8.2% by volume of acetylene, hydrogen, carbon monoxide, cargon dioxide and methane is obtained. The gas mixture used as the starting material must have a composition determined by the upper burning limit. If the oxygen content of the gas mixture is smaller than the above value, only unstable burning takes place, while a higher oxygen content results in the deterioration of the conditions of equilibrurn of acetylene formation. The mixture of methane and oxygen is to be preheated before reaction in order to improve the conditions of acetylene formation and to decrease oxygen consumption. The upper limit of preheating is determined by the ignition temperature of methane (645 C.). By partial oxidation of methane, when the gas mixture is preheated to the temperature of 600 C., allowable from the point of view of safety, and oxygen gas having a purity above 99% is used, the specific oxygen consumption amounts to 5 kg. oxygen per kg. acetylene. The diminution of the purity of the oxygen gas causes a significant increase of the specific oxygen consumption.

According to another important group of cracking processes, the calorific and decomposing reactions are carried out in two steps. These processes are characterized by burning the almost stoichiometric mixture of a selected fuel gas and oxidizing agent, using preferably a mixture consisting of a fuel gas having a high hydrogen content and of oxygen. The fuel gases thus formed, having a temperature of 27002900 C., are cooled to 2400 C. in order to save the structural material; the hydrocarbons to be decomposed are mixed into the heat carrier gas and the reaction is quenched at the appropriate moment in the usual way. As cracking reactions possess a high reaction velocity, the hydrocarbons are to be admixed with the heat carrier gases in an extremely effective way. At the place of admixture both the heat carrier gases and the hydrocarbons are to be circulated with very high flow velocity, generally amounting to the value of the speed of sound and complicated constructions are required. In order to obtain appropriate safety by heating and quick burning, the fuel gas and the gas to be oxidized are also to be admixed with a velocity equivalent to the speed of sound. The above process has considerable disadvantages: the flue gases of high temperature cause several difficulties, the admixture requires high energy consumption, the augmentation of the dimensions result in a more and more non-homogeneous admixture-the above factors decrease the economic efficiency of the process to a significant extent. Moreover, complicated reactors are required to perform the above method.

The present invention provides a process for the thermal decomposition of saturated hydrocarbons containing at most 5 carbon atoms by means of a one-step flame reaction, the heat quantity necessary for thermal decomposition being procured by using fuel gases having a higher burning speed and a upper burning limit than the hydrocarbons to be decomposed. The essential feature of the present invention resides in feeding a mixture of the hydrocarbons to be decomposed, fuel gases and the oxidizing gases into the burner of the flame reactor, in such ratio that, on burning the gas mixture, a reaction temperature of 1000-1600 C. is achieved; at least 30%, but preferably more than of the heat quantity required for the reaction are provided by the partial or total burning of the fuel gases and the decomposition reaction is immediately quenched in the usual way, when the desired unsaturated hydrocarbons have been formed.

Burning becomes generally more stable than the usual partial oxidations, under the effect of the fuel gas added to the hydrocarbons to be decomposed. At the higher decomposing temperature, due to the higher burning velocity of the fuel gases, cracking of the saturated hydrocarbons soon starts. However, it is completed only after the fuel gas used for heat output has been burnt. Heat output is procured in the first place by fuel gases and burning only takes place to a greater extent to the detriment of hydrocarbons, if oxygen remains in the mixture after the fuel gases have been burnt. Thus the conditions of cracking become more uniform than by partial oxidation. On the one part, the yield of unsaturated hydrocarbons becomes higher and, on the other, specific oxygen consumption and soot formation decrease.

Acetylene, ethylene or a mixture of acetylene and ethylene are formed as a principal product of the re action. As a byproduct generally carbon dioxide, carbon monoxide, hydrogen and methane are formed. The

concentration of hydrogen and carbon monoxide formed usually surpasses the amount of fuel gases, required for heat output, so that the byproduct gases obtained by cracking cover the fuel-gas consumption of the process according to the present invention.

The nature and the ratio of the unsaturated hydrocarbons formed by the decomposition of saturated hydrocarbons, particularly that of acetylene and ethylene, are adjusted by the temperature of thermal decomposi tion, the latter being regulated by the ratio of the saturated hydrocarbons, fuel gases and oxidizing gas, depending on the quality thereof.

If the reaction is carried out for a longer period of time than required for acetylene formation and somewhat more oxygen is used, the saturated hydrocarbons decompose completely and synthesis gas free of soot is obtained.

The process of the present invention may be carried out at atmospheric pressure and at smaller or higher pressures too, which may even expand to the vaiue of atmospheres. Thus the reaction conditions and the quality of the raw material may be altered to a significant extent. The reaction conditions are determined by the quality and ratio of the raw material, fuel gas and oxidizing gas. The ratio of acetylene and ethylene depends on the raw material, temperature, and duration of the reaction. The reaction may be carried out also when higher or smaller amounts of fuel gas are admixed with the hydrocarbon to be decomposed, than required for heat output.

It is preferable to use a fuel gas having low oxygenconsumption and providing flue gases, for the reaction mixture, which exhibit a favorable effect on the equilibrium conditions of the decomposition reaction. Thus specific oxygen-consumption, being characteristic of the economy of the process, may be improved to a considerable extent. The requirements according to the present invention with respect to the fuel gases, may be determined from the burning characteristics of the fuel gases; such values of some saturated hydrocarbons, fuel gases and pure oxygen are summarized in the following table.

It has been found that it is preferable by the process of the present invention to pr heat the hydrocarbons, fuel gases and oxidizing gas to a temperature of 400 to 600 C. This may be carried out by preheating the components separately, before the mixing of the same or by preheating the mixture of the hydrocarbons and fuel gases and admixing the same with the preheated oxidizing gas. One may also proceed by preheating the previously prepared mixture of the components.

/lhe data relates to petrol. VCm./sec.-maximal burning velocity Zopercent by volume-upper burning limit.

p-KcaL/normal cubic meter of oxygen-specific heat output related to oxygen. v

1 Without dimensioneificicncy of furnace technique at a reaction temperature 0f1,200 O.

1;Kcal./nor1nal cubic meter of oxygenproduct of multiplication characteristic of the economy of heat output.

According to the data of the above table, hydrogen meets all the requirements with respect to the fuel gases. Moreover, according to experimental data, the steam formed by burning exhibits a favorable effect from the point of view of the decomposition reaction, as it inhibits unfavorable side-reactions, particularly the precipitation of carbon. Carbon monoxide also complies with the requirements, but it may only be used as a fuel gas when admixed with hydrogen, due to the slow burning of carbon monoxide. The admixture of carbon monoxide with d the hydrocarbon to be decomposed is useful, in the first place, because it decreases the carbon monoxide formation in the decomposition reaction, the latter being very uneconomical according to the last column (p77) of the table.

It is to be noted that the known procedures, by which only small amounts of 05-10% by volume of other gasses or vapors, such as hydrogen, carbon monoxide, carbon dioxide, steam, formaldehyde, methanol, are added to the methane raw material, are only directed to promote more or less acetylene formation, but such small amounts of the above gases are completely unsuitable to achieve the effect of a fuel gas, used according to the present invention.

By appropriate addition of the oxidizing gas it is sometimes preferable to carry out the reaction at the upper burning limit, corresponding to the composition of the reaction mixture, under homogenous burning conditions. The concentration of the oxidizing gas in the starting mixture is to be regulated on the basis of the oxygen concentration of the product gas, so that the oxygen content of the residue should amount to a value between 0.2% and 0.5% by volume.

One may also proceed according to the present invention by using low-rate gases of low nitrogen and carbon dioxide content, comprising, apart from hydrogen and carbon monoxide, at least 25% of saturated hydrocarbons containing at most 5 carbon atoms, as a mixture of hydrocarbons and fuel gases.

The process according to the present invention may be carried out in flame reactors generally used, by the partial oxidation of methane. If relatively large amounts of fuel gases are to be admixed with the raw material, it is preferable to modify the equipment by placing two admixture areas before the reactor, in which at first only the fuel gas and then the oxygen are admixed with the raw material to be decomposed.

A further feature of the present invention provides an apparatus for the production of unsaturated hydrocarbons from saturated hydrocarbons containing at most 5 carbon atoms by means of thermal decomposition by flame reaction.

For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings.

In the drawings:

FIGURE 1 represents a longitudinal section of the apparatus according to the present invention, while FIG- USE 2 is a Il-Il section thereof.

The most essential part of the apparatus is the burner according to the present invention. Apparatuses, in which flame reaction processes are carried out, generally consist of the following three parts, as illustrated in the drawings: diffuser 1 distributing the combustible mixture, combustion chamber 3 and section 2 having a restricted free cross-section, separating parts i and 3 from each other. The present invention also relates to a new formation of section 2 having a restricted free cross-section.

In the hitherto known apparatus the free cross-section of section 2 has already been restricted to such an extent that the mixture passes through it with a higher speed than the burning velocity thereof, so that the backfire in diffuser ll could be hindered. The entire cross-section of section 2 was generally filled with a one-piece or split fireproof ceramic block, on the whole cross-section of which regularly distributed slits of circular section Were formed in the direction of flow, as a restricted cross-section. Combustion chamber 3 has such a cross-section that therein the flowing velocity of the mixture is smaller than the burning velocity thereof, and this enables the formation of a stable flame in the combustion chamber. In order to provide stable flame and to hinder backfire in the diffuser, the ratio of the free cross-section of section 2 and that of combustion chamber 3- was adjusted perferably to a value between 1:4 and 1:12, while the diameter of the borings in section 2 were preferably 7 to mm. By the above expedients at the junction of section 2 having a restricted free cross-section, and of combustion chamber 3, so great a dead area is formed that coke deposits occur in the dead area as a result of backflows, and such desposits must often be removed during processing. In a known apparatus scraping was used every two hours. The scraping of coke disturbs on one part, normal processing and, on the other, it causes the early waste of the fireproof material of section 2.

It is also known that flame-reaction processes may be carried out in apparatuses in which the section 2 thereof (having restricted cross-section) is made of metals and cooled with water, in order to save the same from detrimental heat quantities, radiated from the combustion chamber. Such apparatus are of complicated hollow construction, and moreover a large dead area is formed at the junction of sections 2 and 3. Consequently they possess significant disadvantages. According to a further type of the known apparatuses borings of greater and smaller resistance are used in order to form a guard-flame. The above apparatus is however absolutely unsuitable for the carrying out of the process according to the present invention and may only be considered as a disclosure of channel systems of greater or smaller hydraulic resistance.

According to the present invention, the restricted crosssection of section 2 is formed by slots 5, bordered by parallel metal plates 4, having preferably a gauge of 1.5 to 3 mm. The size of slots 5 is determined by spacer ribs 6 formed on the plates and illustrated in FIG. 2. The ratio of the cross-section of slots 5 and that of plates 4 varies between the values of 1:1 and 1:2, thus the ratio of the free cross-section of section 2 and of the cross-section of combustion chamber 3 is considerably smaller than the corresponding ratio in known apparatuses of the same type. The apparatus according to the present invention provides the required safety against backfire in the diffuser and it also produces a stable flame, even with the above ratio, as slots 5 are of such size that, by loadings used, the Reynolds number amounts to a value entering into the laminar range among the slots and into the turbulent range in combustion chamber 3. This is also advantageous, as the burning velocity of the mixture is significantly smaller in the laminar range than in the turbulent range. Thus the ratio of the flow velocity and burning velocity will be greater among the slots, so that the cross-section ratio used in the apparatus according to the present invention procures suflicient safety against backfire in the diffuser.

Safety against backfire in the diffuser depends naturally also on the height of plates 4. The higher the safety intended to be achieved and the greater the pressure-oscillation which may occur in the system, the higher are the plates are to be inserted between the diifuser and the combustion chamber.

The dense distribution of slots 5 having identical crosssections on the cross-section of section 2 provides regular burning and thus preferable reaction conditions too. Dead cross-sections are significantly smaller. Consequently the dead flow area is decreased so that there is no possibility for the formation of coke deposit.

Thermal loading of plates 4 is relatively small, as the ratio of the surface radiated by the combustion chamber related to that cooled by the mixture circulating among the plates is very preferable (it varies between the values of 1:50 and 1:100). Accordingly section 2 may be safely prepared from a metal resistant to a temperature not higher than 100 C. above the temperature of preheating, and no water cooling is required.

A further characteristic feature of the present invention resides in the fact that the restricted cross-section of section 2 is formed from two different channel systems, through which the mixture flows with different velocities.

One channel system, through which the majority of the mixture flows, consists of slots 5 having a relatively large cross-section. Flow resistance is here smaller, and thus high flow velocity may be achieved, which safely hinders the backfire into the diffuser. On the other hand in grooves 7 formed in the spacer ribs and having narrower crosssections, flow-resistance is higher and consequently flow velocity is smaller. By appropriate dimensioning of grooves 7, it may be achieved that the flow velocity of the mixture amounts to a value below the breaking down speed of the flame, but it approaches the burning velocity only to such an extent that there is no backfire in the diffuser through the narrow grooves; the latter being also hindered by the cooling effect of the metal walls. Thus the fact that the mixture flows from grooves 7 into the combustion chamber with a velocity possibly smaller than the burning velocity, but certainly smaller than the breaking down speed of the flame, ensures the continuity of burning, as the mixture flowing out from grooves 7 provides permanent ignition flames for the mixture, flowing out with greater velocity from slots 5, which are in the immediate vicinity of grooves 7. The ignition flames are regularly distributed over the whole cross-section and ensure the stability of burning, while stable burning enables a considerable variation of the loading of the burner as loading may be increased, until the velocity of the mixture, flowing out from grooves 7, remains below the breaking down speed of the flame.

Further details of the present invention are illustrated by the examples. Examples 1 to 3 relate to forms of realization of the process, while Examples 4 and 5 are embodiments of the apparatus, the latter being disclosed in connection with the well-known partial oxidation of methane.

Example 1 60 normal cubic meters per hours of a gas-mixture at atmospheric pressure, preheated to 400 C., having the following composition are fed into the burner of a flame reactor:

Percent by vol. Methane 1 31.8 Ethane 1 3.6 Propane 1 1.5 Hydrogen 2 28.8 Oxygen 27.2 Nitrogen 3 5.3 Carbon dioxide 4 1.8

1 As gas-mixture to be decomposed.

2 As fuel gas.

As contamination of H2.

t As contamination of the hydrocarbons.

On partial burning of the above mixture by the oxygen content of same, a reaction temperature of about 1450 C. was obtained. The reaction took place during 3.10- seconds, whereupon the reaction product was quickly cooled with water to a temperature below C. 65 normal cubic meters per hour of a dry product gas of the following composition were obtained:

Percent by vol. Acetylene 8.2 Carbon monoxide 17.2 Hydrogen 61.2 Methane 4.0 Oxygen 0.3 Nitrogen 4.9 Carbon dioxide 4.2

Thus the use of hydrogen as the fuel gas results in the specific oxygen consumption, characteristic of the economy of the process, is 3.75 g. oxygen per kg. acetylene, while that of the known partial oxidation method with preheating to 600 C. was significantly higher, 5 kg. oxygen per kg. acetylene. The value of 3.75 kg. oxygen per kg. acetylene achieved by the present example may be further decreased if the starting gas mixture contains no nitrogen contamination.

It is to be noted that the above favorable result was obtained by using preheating to only 400 C., which means further savings in the structural material of the preheater and in the heat requirement for preheating.

Example 2 60 normal cubic meters at a gas mixture of atmospheric pressure and preheated to 400 C., having the following composition are fed into the burner of flame reactor:

Percent by volume Methane 1 30.5 Carbon monoxide 2 15.3 Hydrogen 2 32.0 Oxygen 22.0 Nitrogen 3 0.2

1 As gas to be decomposed.

3 As fuel gas.

3 As contamination of the oxygen.

The above gas mixture was partially burnt by the oxygen content thereof under the reaction conditions described in Example 1. 52.5 normal cubic meters per hour of a dry gas mixture of the following composition were obtained:

Percent by volume Acetylene 9.3 Carbon monoxide 26.1

Hydrogen 56.5 Methane 3.5

Oxygen 0.25 Nitrogen 0.25 Carbon dioxide 4.1

According to the present example, if an approximately 1:2 mixture of carbon monoxide and hydrogen is used as a fuel gas, and oxygen having a purity of 99% is used as the oxidizing gas, specific oxygen consumption decreased to the value of 3.3 kg. oxygen per kg. acetylene.

Example 3 60 normal cubic meters per hour of a gas mixture at atmospheric pressure, preheated to 400 C. and having the following composition were fed into the burner of a flame reactor:

Percent by volume Methane 1 20.4 Carbon monoxide 2 20.4

Hydrogen 2 42.5 Oxygen 16.5 Nitrogen 3 0.2

1 As gas to be decomposed.

2 AS fuel gas.

3 As contamination of the oxygen.

The gas mixture was partially burnt by the oxygen content thereof, according to the reaction conditions described in Example 2. 47.1 normal cubic meters per hour of a dry gas mixture, having the same composition as that prepared according to Example 2, were obtained.

The specific oxygen consumption thus obtained amounts to 2.76 kg. oxygen per kg. acetylene. Thus the increase of the introduced amount of the fuel gas (an approximately 1:2 mixture of carbon monoxide and hydrogen) at the expense of methane, resulted a further reduction of the specific oxygen consumption.

It is to be noted that the total amount of the introduced carbon monoxide and hydrogen gases is somewhat smaller than that of the carbon monoxide and hydrogen gases formed by the reaction, thus the carbon monoxide, hydrogen and methane components of the product gases may be directly used as starting material of the reaction.

Example 4 A mixture of 62.5% of methane and 37.5% of oxygen, preheated to 600 C. and having a pressure of 1:1 atm. were passed through the burner. Specific loading related to the restricted free cross-section of the burner amounted to 200 normal cubic meters of methane per square decirmeter hour. The height of plates 4 amounted to 75 mm. the highest temperature thereof on their side toward the combustion chamber was 625 C. No coke deposit was observed on the plates. On quenching the reaction in the usual way, a product-gas of the following composition was obtained:

Percent by volume Acetylene c 8.6 Carbon monoxide 26.2

Hydrogen 57.5 Carbon dioxide 3.7

Methane 3.5

Ethylene 0.5 Oxygen 0.2

Example 5 A gas mixture consisting of 63% of methane and 37% of oxygen, preheated to 600 C. and having a pressure of 4 atm. was passed through the burner. Specific loading, related to the free cross-section of the burner increased to 600 normal cubic meters of methane per square decimeter hour, due to the higher pressure. The height of the plates amounted to 75 mm. according to this example also. However, the highest temperature of the plates was only 650 C. on their side toward the combustion chamber. No coke deposit was observed on the plates.

Data according to Examples 4 and 5 relate to experiments carried out with the apparatus illustrated in the drawings, provided with spacer ribs 6 and grooves '7. If the spacer elements and grooves '7 are formed from plates, being supported against plates 4, being thinner than the latter and in some places recessed according to the breadth of the slot in the direction of the flow, or if they are formed from separately inserted tubes of small diameter, similar results are then obtained.

What we claim is:

1. An apparatus for the flame reaction of a combustible gas mixture comprising, in combination, a diffuser through which the gaseous mixture to be burned flows longitudinally; a combustion chamber wherein said gaseous mixture from said diffuser is burned; an intermediate section connecting said diffuser to said combustion chamber; said intermediate section having a plurality of spaced parallel plates extending longitudinally between said diffuser and said combustion chamber and formed from a material of high thermal conductivity, said spaced plates defining longitudinally extending channels parallel to the direction of flow of said gaseous mixture and restricting the area for flow of said gaseous mixture from said diffuser into said combustion chamber; and elements contained in said channels and further restricting the area for flow of gas through said channels; each of said elements forming a restricted flow passage between said difluser and said combustion chamber; the ratio of the lateral width of said channels to the thickness of said plates ranging from 1:1 to about 1:2, and the lateral width of said channels being of a dimension such as to provide a Reynolds number, for the flow of gas through said channels, in a laminar range and a Reynolds number, for the flow of gas in the combustion chamber, in the turbulent range.

2. An apparatus, as claimed in claim 1, wherein said elements extend from one of the plates forming said channels into the flow of gaseous mixture through said channels, with said elements being longitudinally recessed at their outer surfaces to form said restricted flow passages.

3. An apparatus, as claimed in claim 1, wherein said elements are formed from said plates and extend into the flow of said gas through said channels.

4. An apparatus, as claimed in claim 1, wherein the plates have a thickness of from 1.5 to 3 mm. and a length of from about 70 to mm.

9 10 5. An apparatus, as claimed in claim 1, wherein said References Cited elements are formed from each of said plates and ex- UNITED STATES PATENTS tend into the flow of gaseous mixture through said chan- 2 664 4 5 0 12/1953 Sachsse et a1 260 679 nel, with each restricted flow passage being formed be- 2833839 4/1958 Lehrer tween the element and the wall of the plate adjacent said 5 P1ate from which Said element extends- JAMES H. TAYMAN, JR., Primary Examiner.

6. An apparatus, as claimed in claim 1, wherein said U S Cl X R elements are separately installed ribs extending into the flow of gas through said channels. 260-679; 23-284; 158112, 116 

